One of the most effective and efficient methods of deploying high-speed digital services to business and residential customers may be to use one of the many forms of Digital Subscriber Loop (DSL) technologies over copper telephone wires. This approach has become very popular in the last 20 years due to the fact that copper wires are already deployed almost everywhere and are easy to access, both at a Central Office (CO) and at a Remote Terminal (RT) or at a customer premises location.
However, one limitation of DSL technology is that the data capacity of a copper wire pair decreases significantly as the length of that copper wire pair increases. Therefore, customers located more than a few kilometers from a Central Office may not be provided with high data speeds over copper wires.
One way to mitigate this problem may be to use multiple copper pairs to each customer premises location, thereby increasing a total data rate of a resulting multi-pair copper link. This method is often referred to as “bonding” of copper pairs.
Another method for extending reach of DSL services may be to utilize repeaters. Repeaters may be installed in intermediate locations on a copper loop, and may contain one or more transceivers that may receive and re-transmit a signal from and to neighboring devices, including other repeaters. A resulting repeatered copper link may comprise multiple shorter segments that may be connected to each other via repeaters. As a result, the capacity of an original long copper loop may be increased to the capacity of the longest of these multiple shorter segments.
Repeaters may also be combined with bonding to further increase rate and reach of DSL services by using, for example, multiple copper pairs, each of which may be partitioned into multiple repeatered segments.
Deployment of repeaters may face many operational challenges including, but not limited to, installation procedures, proper electrical grounding and shielding, providing power over copper wires, and remote troubleshooting and management to avoid the need to dispatch a technician to a field location every time a problem occurs with one or more of the repeaters.
Another reason that repeaters may not be widely used may be their potential for generating significant spectral interference, for example, to residential DSL services, which have become ubiquitous in the last decade. Spectral interference between different high-bitrate services in a copper binder may be caused by each copper pair acting as an antenna. A signal transmitted on each copper pair, which may be intended for a receiver located at another end of that copper pair, may also inadvertently be received by one or more neighboring copper pairs, because those pairs may not individually be shielded from each other. This phenomenon is often referred to as “crosstalk”, aptly named for an effect observed in the early days of the telephone, when a telephone discussion taking place on one line could sometimes be overheard by people conversing on a different line.
Due to some of the physical characteristics of copper pairs, and in particular due to the typical length of the twist between two copper wires making up a copper pair, crosstalk coupling between different pairs may increase dramatically with frequency. But this crosstalk coupling is only one of multiple factors that determine the severity of crosstalk; other determining factors include the power level of the disturbing transmitter and the sensitivity of the disturbed receiver at any given frequency. For example, if a transmitter is transmitting in one frequency band and a nearby receiver is receiving in a completely different frequency band, then there may be almost no crosstalk from this particular transmitter to this particular receiver.
Early repeaters have used Alternate Mark Inversion (AMI) or High Density Bipolar order 3 (HDB3) line codes to deliver T1 (1.544 Mbps) or E1 (2.048 Mbps) services over longer copper loops. These technologies made inefficient use of frequency bands, utilizing almost 2 MHz of frequency spectrum to deliver a mere 1.544 or 2.048 Mbps over 2 copper pairs at distances no longer than 1-1.5 km. Later on, symmetric DSL standards such as HDSL (High-speed DSL), HDSL2, HDSL4 and Single-pair High-speed DSL (SHDSL) allowed deployment of the same T1/E1 services over repeatered copper links while making more efficient use of a frequency spectrum and reducing the number of repeater locations and repeatered segments needed to deploy a repeated copper link compared with AMI/HDB3 methods.
Despite these advancements in repeater technology, the proliferation of residential DSL services may have limited potential increases in deployment of repeaters. One reason for this may be that repeaters typically generate much stronger crosstalk into residential DSL receivers than non-repeatered services deployed from a Central Office (CO). As repeaters may be placed much closer to remotely located residential DSL receivers, their interfering signal may encounter much lower attenuation on a short path to those residential DSL receivers that may be in nearby locations, and this interfering signal may be much stronger when it reaches those residential DSL receivers. Since this strong signal may overlap in frequency with a downstream signal of residential DSL services, it may cause significant spectral interference to those affected residential DSL receivers in nearby locations.
As a result, various countries have imposed significant restrictions on the deployment of repeaters in outside loop plants. For example, the American National Standards Institute (ANSI) has issued recommendation T1.417, which specifies that repeaters should only be deployed in North America with a line bitrate of 776/784 kbps per copper pair when used with HDSL4 technology, or a maximum line bitrate of 634 kbps per pair when used with SHDSL technology. This restriction is designed to reduce the upstream frequency band of disturbing signals to approximately 130 kHz, in order to minimize its overlap with the downstream frequency band of residential DSL services, which typically starts at about 140 kHz. Other countries have imposed similar restrictions, for example, some European countries limit the bitrate of repeaters to 1 Mbps per copper pair.
These restrictions may have limited the use of repeaters for delivery of high-speed data services. For example, consider the problem of wireless backhaul, which may involve providing a high-speed communications link between a Central Office and wireless basestations, so that these basestations may effectively provide high-speed data services to wireless subscribers. In a particular case where a required bitrate for a backhaul link may be 20 Mbps, and a wireless basestation may be 10 km away from a Central Office, so that repeaters may be required, it may take 26 copper pairs to deliver this service under a restriction of 768 kbps per pair, and 32 pairs under a restriction of 634 kbps per pair. Clearly, utilizing that many copper pairs for one 20 Mbps link may be impractical and expensive.
Therefore, it may be highly desirable to deploy repeaters with significantly higher bitrates per copper pair while ensuring that these repeaters do not generate significant levels of spectral interference to residential DSL services.